The invention relates to a pentagonal roof prism for a viewfinder, and more particularly, to such a prism for a viewfinder which is used in a single lens reflex camera or other optical instruments.
A single lens reflex camera or other optical instruments incorporate a variety of viewfinders which are sophisticated in one way or another in order to reduce the overall size, to facilitate the manufacture or to reduce the manufacturing cost.
As is well recognized, a single lens reflex camera includes an optical system for its viewfinder which is constructed as illustrated in FIG. 1. Referring to FIG. 1, light 7 from an object being photographed which is passed through a lens 1 is reflected upward by a movable reflecting mirror 5 to be focused onto a focusing screen 2, disposed at a point which is conjugate to the focal plane of the lens 1, as an erect image which is laterally reversed. The image formed on the focusing screen 2 can be viewed through an eyepiece 8, pentagonal roof prism 4 (which will be hereafter referred to as a pentaprism) and a field lens 3. The pentaprism 4 causes the image to be reflected twice by its reflecting surfaces, including the top surface, to return the optical axis to a horizontal path and reversing the lateral side, thus producing an erect image with correct lateral orientation. Hence, the erect image can be viewed through the eyepiece 8.
In FIG 1, the distance between the optical axis 1a of the lens 1 and the top apex 4a of the pentaprism 4 is designated by character D, and it is known that a compact single lens reflex camera having a reduced overall size can be produced by arranging an optical system for the viewfinder in which the distance D is minimized. At this end, the field lens 3 is formed integrally with the bottom surface 4b of the pentaprism 4, thus reducing the magnitude of distance D by an amount equal to the separation which was originally present between the pentaprism 4 and the field lens 3. However, this is insufficient, and there is still a strong desire for a further reduction in size. In addition, the presence of the convex surface of the field lens 3 intermediate the focusing screen 2 and the field lens 3 requires a definite spacing therefor. A further improvement is required in this region to reduce the distance D to a lesser value.